Purpose <p>Gadolinium-neutron capture therapy (Gd-NCT) employs isotopically enriched Gadolinium (Gd) and thermal neutrons to selectively target cancer cells. This study investigated the targeting efficacy of ¹⁵⁷Gd-DOTA-PSMA (Prostate-Specific Membrane Antigen) in prostate cancer and explored its potential applications in Gd-NCT.</p> Methods and results <p>We developed ¹⁵⁷Gd-DOTA-PSMA, a novel theranostic bio-gadolinium agent specifically designed for magnetic resonance imaging (MRI)-guided Gd-NCT. ⁶⁸&#xa0;Ga-DOTA-PSMA positron emission tomography-computed tomography (PET/CT) imaging showed peak radiotracer uptake at 2&#xa0;h post-injection, with a tumor-to-non-tumor (T/NT) ratio of 6.95 ± 0.60. MRI analysis confirmed a stable T₁ signal enhancement 2&#xa0;h post-injection. Time-of-flight inductively coupled plasma mass spectrometry (TOF-ICP-MS) revealed significantly elevated Gd concentrations in 22Rv1 tumor compared to PC-3 tumor and other healthy organs. ICP-MS analysis showed Gd concentrations of 165.69&#xa0;μg [Gd]/g in 22Rv1 tumors and 35.25&#xa0;μg [Gd]/g in blood, yielding a tumor-to-blood (T/B) ratio of 4.65 ± 0.54 and a T/NT ratio of 3.65 ± 0.49. Neutron irradiation with ¹⁵⁷Gd-DOTA-PSMA reduced cell viability, inhibited colony formation, and induced DNA damage and apoptosis in 22Rv1 cells. In 22Rv1 mice, γ-H2AX levels peaked at 6&#xa0;h post-irradiation, accompanied by an increase in pro-apoptotic proteins and a decrease in anti-apoptotic proteins over 24&#xa0;h. In the NCT group following the injection of ¹⁵⁷Gd-DOTA-PSMA, there was effective suppression of tumor growth without a loss of body weight, resulting in a 1.7-fold increase in median survival compared to control group.</p> Conclusions <p>¹⁵⁷Gd-DOTA-PSMA, as a theranostic bio-gadolinium agent designed for targeted Gd-NCT in prostate cancer, represents a novel therapeutic approach and broadens the scope of potential applications of neutron capture therapy.</p> Graphical abstract <p></p>